1. Field of the Invention
The present invention relates to an apparatus for controlling exhaust gas recirculation (EGR) in a diesel engine.
2. Description of the Prior Art
In a diesel engine, EGR is used for reducing the nitrogen oxides (NOx) in exhaust gas. Excessive EGR, however, generates black smoke, therefore EGR should desirably be controlled finely in response to the engine speed and load.
A known EGR system enabling fine control of the amount of EGR is described below. The system uses a well known diaphragm type EGR valve and supplies to the diaphragm chamber of the EGR valve a vacuum adjusted for the most desirable amount of EGR for the engine speed and load.
More specifically vacuum from a pump is adjusted to a constant vacuum by a constant-pressure valve then supplied to the diaphragm chamber through an electromagnetic valve. An orifice leading to the atmosphere is provided in at least one portion of the wall sustaining the above-mentioned pressure. The amount of air passing through the orifice is controlled by the opening degree of the electromagnetic valve for control of the pressure of the diaphragm chamber of the EGR valve.
The opening degree of the electromagnetic valve is controlled by the ratio of the conducting time to nonconducting time of the electric current applied thereto. The electric current is turned on and off by the output of an electronic circuit.
The diameter of the orifice is an important factor in achieving precise control in this system. Too large a diameter of the orifice would reduce the range of opening degree of the electromagnetic valve able to control the amount of EGR. Conversely, reduction of the diameter of the orifice to one assuring a certain range of control would result in the EGR valve not being able to respond quickly enough to rapid changes in the driving conditions, leading to inappropriate EGR amounts. As a result, when the engine load increases rapidly, the excessive EGR would cause a great amount of the black smoke.
To deal with this problem, the diameter of the orifice has been set to one with a finely EGR controllable size. Rapid changes in the engine load have been detected so as to supply electric current to the electromagnetic valve, provided in the bypass path of the orifice, to open the path and instantly return the diaphragm chamber of the EGR valve to atmospheric pressure, thereby improving the response of the EGR valve and preventing the exhaust of black smoke due to the excess of the amount of EGR. The above-mentioned system, however, requires a vacuum source, a constant-pressure valve, an orifice, an electromagnetic valve controlled by an electric signal having a duty cycle, an electromagnetic valve provided in the bypass path, an EGR valve, a sensor for the lever opening degree, a sensor of the engine speed, and a control circuit. Such an EGR system therefore suffers from the demerit of too many parts.
The present invention is proposed in order to eliminate the above-mentioned demerit.